A goal of CT designers is to obtain rapid scanning with improved resolution and contrast discrimination in order to obtain clear understandable images substantially free of artifacts. The rapidity of obtaining images is important for minimizing artifacts caused by the movement of the patient or the movement of the patient's organs during the scan. Other advantages of shortened time are less patient inconvenience and increased patient throughput.
The early CT equipment used a single beam operating in translate-rotate motion. The X-ray tube and the detectors were arranged a fixed distance from each other at opposite sides of the patient. The detected x-ray beam intensity was recorded as the beam translated across the patient at a single angular position. The angular position was changed and the beam was again translated across the patient. This operation was repeated a number of times. Obviously with this method it took a relatively long time to complete each scan (approximately 5 minutes). The time was shortened appreciably, to approximately 10 seconds, by using multiple detectors in a fan beam.
The next step in the development of computed tomography was to eliminate the translation motion during scanning. This was accomplished by using a relatively wide arrays of detectors in a fan beam of x-rays that covered the width of the scanned object. Both the radiation source and the detectors were rotated (Rotate-Rotate or R/R CT Scanner). The resulting scan time was cut to a period of 2 to 10 seconds.
Although a rotation of only 180 degrees (plus the fan beam angle) are needed to obtain sufficient data for reconstruction, the R/R scanners generally rotate through 360 degrees or more per scan to compensate for divergence of the beam in direction perpendicular to the fan beam so as to enable the use of a small size radiation source and none-the-less obtain more uniform thickness in the resulting section of the patient. The reduced size radiation source also provides a more sharply collimated beam and consequently reduces radiation dosage. In addition, the 360 degree rotation reduces skin dosage.
Since the inception of R/R type CT scanners, however, the image has been plagued by ring artifacts caused by inbalances between detector elements. The designers of the scanners using the R/R systems have resorted to many different complicated systems to eliminate or at least minimize the occurence of ring artifacts. (See the paper entitled "Suppression of Ring Artifacts in CT Fan-Beam Scanners" by G. Kowalski published in IEEE Transactions on Nuclear Science, Vol. NS-25, No. 5, October 1978; and U.S. Pat. No. 4,323,784).
Among the prior art solutions of the ring artifact problems are the use of special matched detector arrays for example, the use of matched gaseous ionization detectors with circuitry for maintaining the match (See, for example, U.S. Pat. No. 4,334,154). Special scanning sequences are also used wherein complicated calibrations are performed within the scan period to measure and correct the detectors on-line. These solutions each have inherent disadvantages. For examples the gaseous ionization detectors have low stopping power and therefore an increase x-ray dose is required for a given image quality. The special scanning sequence usually requires a pulsed radiation source with the consequent high priced power supply and x-ray tube. In addition special periodic calibrations are required for the detector array; which complicates the use of the system. Therefore, a relatively simple, inexpensive method of eliminating ring artifacts when using R/R equipment with unbalanced detector arrays is sorely needed.